cisco express forwarding wayne lewis may11
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© 2010 Cisco Systems, Inc. All rights reserved. 1
Cisco Express Forwarding
Wayne Lewis, Ph.D. Cisco Networking Academy Instructor and Cisco Press Author Pacific Center for Advanced Technology Training Honolulu Community College
Virtual Academy Conference 2010
2© 2010 Cisco Systems, Inc. All rights reserved.
Agenda
Cisco Express Forwarding (CEF) Overview
Layer 3 Switching
What Is CEF?
How Does CEF Work?
How Do You Configure CEF?
3© 2010 Cisco Systems, Inc. All rights reserved.
Cisco Express Forwarding Overview
CEF is state-of-the-art technology used on Cisco devices that offloads traditional CPU duties to dedicated application specific integrated circuits and data structures to increase the speed of switching packets on Cisco routers and switches.
Formally, CEF is a scalable, distributed, layer 3 switching solution designed to meet the future performance requirements of the Internet and enterprise networks. It represents the latest advance in Cisco IOS switching capabilities. CEF is also a key component of Cisco's MPLS implementation.
4© 2010 Cisco Systems, Inc. All rights reserved.
Layer 3 Switch Processing
In Layer 3 switches, the control path and data path are relatively independent.
– The control path code, such as routing protocols, runs on the route processor.
– Data packets are forwarded by the switching fabric.
5© 2010 Cisco Systems, Inc. All rights reserved.
Layer 3 Switch Processing (Cont.)
Layer 3 switching takes place using one of these two methods:
1. Route caching: A Layer 3 route cache is built in hardware as the switch sees traffic flow into the switch.
2. Topology-based switching: Information from the routing table is used to populate the route cache, regardless of traffic.
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CEF-Based Multilayer Switches
CEF caches routing information in the FIB table and Layer 2 next-hop addresses in the adjacency table.
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Multilayer Switch Packet Forwarding Process
Some IP packets cannot be processed in hardware.
If an IP packet cannot be processed in hardware, it is processed by the Layer 3 engine.
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CEF-Based MLS Lookups
1. Layer 3 packets initiate TCAM lookup.
2. The longest match returns adjacency with rewrite information.
3. The packet is rewritten per adjacency information and forwarded.
9© 2010 Cisco Systems, Inc. All rights reserved.
ARP Throttling
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CEF-Based MLS Operation
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Configuring and Verifying CEF
Configuring CEF
ip cef (enabled by default)
ip route-cache cef (VLAN interface on switch, physical interface on router)
Verifying CEF
show ip cef fa 0/1 detail
show adjacency fa 0/1 detail
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The commands required to enable CEF are platform dependent:
Enabling CEF
Swouter(config)#ip cef [distributed]
Swouter(config-if)#ip route-cache cef
On a multilayer Cisco switch or a Cisco router
On a multilayer Cisco switch SVI or Cisco router interface
13© 2010 Cisco Systems, Inc. All rights reserved.
Verifying CEF
Switch#show ip cef [type mod/port | vlan_interface] [detail]
Switch# show ip cef vlan 11 detail
IP CEF with switching (Table Version 11), flags=0x0
10 routes, 0 reresolve, 0 unresolved (0 old, 0 new), peak 0
13 leaves, 12 nodes, 14248 bytes, 14 inserts, 1 invalidations
0 load sharing elements, 0 bytes, 0 references
universal per-destination load sharing algorithm, id 4B936A24
2(0) CEF resets, 0 revisions of existing leaves
Resolution Timer: Exponential (currently 1s, peak 1s)
0 in-place/0 aborted modifications
refcounts: 1061 leaf, 1052 node
Table epoch: 0 (13 entries at this epoch)
172.16.11.0/24, version 6, epoch 0, attached, connected
0 packets, 0 bytes
via Vlan11, 0 dependencies
valid glean adjacency
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Common CEF Problems
Is ideal switching method (CEF, DCEF) in use?
Are CEF tables complete and accurate?
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Verify Layer 3 Switching
Switch#show interface {{type mod/port} | {port-channel
number}} | begin L3
Switch#show interface fastethernet 3/3 | begin L3
L3 in Switched: ucast: 0 pkt, 0 bytes - mcast: 12 pkt, 778 bytes mcast
L3 out Switched: ucast: 0 pkt, 0 bytes - mcast: 0 pkt, 0 bytes
4046399 packets input, 349370039 bytes, 0 no buffer
Received 3795255 broadcasts, 2 runts, 0 giants, 0 throttles
.....
Switch#
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Displaying Hardware Layer 3 Switching Statistics
Switch#show interfaces {{type mod/port} | {port-channel
number}} include switched
Switch#show interfaces gigabitethernet 9/5 | include switched
L2 Switched: ucast: 8199 pkt, 1362060 bytes - mcast: 6980 pkt, 371952 bytes
L3 in Switched: ucast: 3045 pkt, 742761 bytes - mcast: 0 pkt, 0 bytes mcast
L3 out Switched: ucast: 2975 pkt, 693411 bytes - mcast: 0 pkt, 0 bytes
17© 2010 Cisco Systems, Inc. All rights reserved.
Adjacency Information
Switch#show adjacency [{{type mod/port} |
{port-channel number}} | detail | internal | summary]
Switch#show adjacency gigabitethernet 9/5 detail
Protocol Interface Address
IP GigabitEthernet9/5 172.20.53.206(11)
504 packets, 6110 bytes
00605C865B82
000164F83FA50800
ARP 03:49:31
18© 2010 Cisco Systems, Inc. All rights reserved.
Debugging CEF Operations
Switch#debug ip cef {drops | access-list | receive |
events | prefix-ipc | table}
• Displays debug information for CEF
Switch#debug ip cef {ipc | interface-ipc}
• Displays debug information related to IPC in CEF
Switch#ping ip
• Performs an extended ping
19© 2010 Cisco Systems, Inc. All rights reserved.
How Do You Get CEF?
Buy a Cisco router or switch! It’s already there!
20© 2010 Cisco Systems, Inc. All rights reserved.
Summary
Layer 3 switching is high-performance packet switching in hardware.
MLS functionality can be implemented through CEF.
CEF uses tables in hardware to forward packets.
Specific commands are used to enable and verify
CEF operations.
Commands to enable CEF are platform dependent.
CEF problems can be matched to specific solutions.
Specific commands are used to troubleshoot and solve CEF problems.
Ordered steps assist in troubleshooting CEF-based problems.
21© 2010 Cisco Systems, Inc. All rights reserved.
Additional Resources
Basically, Cisco.com!
Get a guest account on Cisco.com (free)
www.cisco.com/en/US/partner/tech/tk827/tk831/tk102/tsd_technology_support_sub-protocol_homeMain page for CEF on Cisco.com
New Cisco Academy CCNP Switch course
22© 2010 Cisco Systems, Inc. All rights reserved.
23© 2010 Cisco Systems, Inc. All rights reserved.
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